Here are the top 10 reasons:

Community edition is most popular and suitable for small business. This has the largest and most active developer community for the E-commerce platform.
Magento is free to download and install. Through Magento Connect Marketplace, you can install extensions and customize accordingly. The platform gets upgraded regularly by the community and security flaws are rectified.
The best part is the extensive community that provides free troubleshooting.

2. Made for E-commerce

Magento, unlike other CMS platforms like wordpress, was created to be an E-commerce platform. The core features of Magento makes building an E-commerce site an ease.
Some of the Magento features are:

Most CMS platforms have core functionality but not all solutions are within the system. To make life easier, plugging in ready-made apps or plugins that integrate new features into the website is preferred rather than coding each new feature from scratch.
While with others, integrating third-party apps is entirely in the control of the service provider, Magento allows you to connect to any kind of app or plug-in easily. Whether it is payment gateways, web analytics or shipping & tracking, Magento can plug-in app of your choice in no time.

4. Modular and Customizable

Each business that uses CMS platforms has different needs and thus, have to use CMS differently. What works for a fashion industry may not work for a furniture industry.

Being an open source, Magento is inherently highly customizable and modular. Its modular elements allow you to develop stunning front-end experience by modifying or plugging in layouts and templates of your choice.

You can easily override or update Layout XML. Backend workflows can be easily redesigned to suit a business by rewriting codes, modifying events and grids.

5. Multiple Websites With One Common Backend System

Typically E-commerce websites, open source or otherwise, have one store per system. This isn’t the case with Magento as you can run multiple websites on the same common backend system.

Magento allows users to create separate and independent websites for the products, each with distinct layouts. You can have different domains for each site. This is controlled simultaneously through a single unified admin panel. This capability is a boon for E-commerce operating in multiple niche categories.

6. Web Services API

Magento Web Services API Provided you facility to manage various magento functions with API Calls. You can provide various different services to your clients, like white label websites with inventory and pricing from your store.

The APIs can perform a wide array of tasks like:

• Create a shopping app for mobile device.
• It can integrate with CRM or ERP back-end systems.
• Integrate with a CMS (Content Management System).
• This also creates JavaScript widgets in the Magento storefront. The widget makes AJAX calls to access services.

7. Flexible and Scalable

E-commerce websites designed on Magento platform adapts seamlessly to new extensions or customized features.

As you further expand and grow the number of products, pages, static blocks, and widgets Magento will grow with you at no additional expense.

8. Built for SEO

Magento is one of the leading E-commerce platforms that are SEO friendly. This not only hosts all your information but also aids your website so it can show up in the search engine space.